1. Field of the Invention
This invention relates generally to bubble removal in uncured liquid adhesives and more particularly to the removal of bubbles or voids formed in the uncured liquid adhesive injected in a fiber optic termination or connector during the adhesive injection process.
2. Description of the Related Art
Most fiber optic assemblies are fabricated by bonding a connector or termination to an optical fiber with an adhesive. Bubbles and voids are introduced into the adhesive and the termination during adhesive mixing, adhesive applicator filling, and termination filler processes. Bubbles and voids in uncured adhesives can be responsible for optical fiber cracks developed during curing processes. In such an instance, the bubbles or voids expand as heat is applied. This can force out an excessive amount of adhesive at the termination end. Too much adhesive surrounding a protruding optical fiber at the termination end can cause some fibers to break during a subsequent heat curing process. It has been found that bubbles and voids in the cured adhesive are strongly correlated with subsequent fiber breakage in the termination, particular after the assembly is exposed to temperature cycling. As a result, product yield is reduced and manufacturing time and cost increased due to scrapped parts and retermination efforts.
Bubbles are generally removed from an uncured adhesive after it is mixed or after an applicator is filled with the mixed adhesive. There are two known techniques for removing bubbles or gas from uncured adhesives, namely: the centrifuge method and the vacuum chamber method. The centrifuge method involves placing the applicator or sealed mixing package containing uncured liquid adhesive in a centrifuge and subjecting it to a spin cycle. Currently there are no known techniques for removing bubbles from a connector or termination using a centrifuge after the liquid adhesive is injected into it. In the vacuum chamber method, the elements are subjected to a vacuum of a predetermined magnitude and duration to draw out the bubbles.
The use of a vacuum to remove bubbles or gas from an uncured adhesive may also be accompanied by the removal of volatile components or solvents from the adhesive. In some instances, the removal of such components may adversely affect the chemistry of the adhesive and thus change it or make it unreliable. Some of the adhesive properties that can change or degrade are: pot life, hardness, glass transition temperature and bond strength. Vacuum chambers are also cumbersome, inefficient, time consuming to use and costly.
Fiber cracking problems are also sometimes caused by the fabrication process sequence, where an optical fiber is bonded to a connector or termination using a shrinkable tube. The shrinkable tube or some other component is applied to the assembly before the liquid adhesive is heat cured. Unfortunately, the processes which use these shrinkable tubes or other components to hold the termination on the optical fiber prior to adhesive curing merely end up forcing the liquid adhesive out the other side from the end of the termination. Bubbles or voids present in the adhesive expand when heated. If not allowed to escape or be vented from the assembly, they also force the adhesive out of the end of the termination. As noted above, too much adhesive surrounding a protruding optical fiber at the termination end can also cause the fiber to break during the heat curing process. Accordingly, the removal of bubbles must be carried out prior to curing, i.e. being subjected to the application of heat.